Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B33/00—Silicon; Compounds thereof
  • C01B33/113—Silicon oxides; Hydrates thereof
  • C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
  • C01B33/18—Preparation of finely divided silica neither in sol nor in gel form; After-treatment thereof
  • C01B33/187—Preparation of finely divided silica neither in sol nor in gel form; After-treatment thereof by acidic treatment of silicates
  • C01B33/193—Preparation of finely divided silica neither in sol nor in gel form; After-treatment thereof by acidic treatment of silicates of aqueous solutions of silicates

Definitions

• the present invention relates to a process for the preparation of finely divided precipitated silica. More particularly the present invention relates to step wise neutralization of soluble alkali metal silicates with acidifying agents viz. mineral acid and acidic silica sol in order to obtain silica having more than 75% particles finer than 10 microns ( ⁇ m).
• Silica is useful in reinforcing elastomers such as rubbers and in improving the resistance of rubbers to abrasion.
• the high structured precipitated silica having fine particle size is particularly used as antiblocking agent for polypropylene and polyethylene films having very high transparency. They are also used as a carrier for dentally active substances, which are stored at the site of action and then release the active substance in small doses over a relatively long period of time (deposition effect, controlled release).
• the silica acts as active substance stores which contain the active substance in absorbed, adsorbed or chemisorbed form.
• Silica is also used in catalysis, inks, papers and in the food industry.
• the resultant precipitated silica and/or silicate particles produced including those that surprisingly exhibit extremely high transmittance properties, thereby potentially providing excellent narrow particle distribution silica materials for clear (transparent) end-uses.
• This process involves post-synthesis treatment to get the narrow particle distribution using a hydraulic chamber press-filtering step combined with vacuum dewatering. This will add to the cost of production.
• US Patent (Pub.) No. US 2005/0228106 A1, (Oct. 13, 2005) assigned to Schaefer et al. describes a method for the preparation of readily dispersible precipitated silica with primary particles of about 5 nm radius, having BET specific surface in the range of 280-350 m 2 /g and bulk density in the range of 0.4 to 0.6 g/cc.
• the precipitated silica in this invention is obtained by neutralization of sodium silicate solution in different steps under reduced ionic strength and at elevated temperature in the range of 60 to 100° C.
• the neutralizing agents used are sulfuric acid and silicic acid.
• This process involves surface modification of primary particles and aggregates by organo-silanes in order to prevent agglomeration of the aggregates to obtain hydrophobic surface with size of the particles in the colloidal range.
• the primary particles are measured by small angle X-ray scattering (SAXS) technique; however no mention has been made for the size of secondary (aggregates) or tertiary (agglomerates) particles.
• SAXS small angle X-ray scattering
• silicic acid in one step of the process makes process complicated, as silicic acid is prepared by passing the silicate solution through a column of cation exchange resin and this solution is unstable and cannot be stored for the longer time. Silicic acid thus obtained should be used immediately and can be stored at lower temperature for few hours only. Another drawback is that the reaction time is longer and it takes about 150 to 200 minutes for complete neutralization.
• U.S. Pat. No. 6,902,715 (Jun. 7, 2005) assigned to Maus, et al. describes an invention which provides silica particles, which include the following physical properties: BET surface area: 100-700 m 2 /g; DBP absorption: 100-500 g/100 g; tamped density: 100-250 g/l; ALPINE sieve residue>63 ⁇ : ⁇ 5%; and particle sizes (cumulative volume distribution): d 95 ⁇ 40 ⁇ m; d 50 ⁇ 20 ⁇ m; and d 5 ⁇ 10 ⁇ m.
• the present invention also provides a process for producing silica having a narrow particle size distribution, which includes drying a silica suspension in a pulse combustion dryer to produce silica particles having particle size distribution (cumulative volume distribution): d 5 ⁇ 10 ⁇ m; d 50 ⁇ 20 ⁇ m; and d 95 ⁇ 40 ⁇ m.
• fine size particles are achieved by post-synthesis treatment, which required special type of pulse combustion dryer. This may add to the cost of production.
• US Patent Application No. 20030118500 (2003) assigned to Chevallier, et al. discloses a process for the preparation of precipitated silica, comprising the reaction of a silicate with an acidifying agent, whereby a suspension of precipitated silica is obtained, followed by separation and drying of this suspension, characterized in that the precipitation is carried out in the following way: (i) an initial stock solution is formed containing at least some of the total amount of the silicate used in the reaction and at least one electrolyte, the concentration of silicate (expressed as SiO 2 ) in the said initial stock solution being between 50 and 60 g/l, (ii) the acidifying agent is added to the said stock solution until a pH of between 7 and 8.5 for the reaction medium is obtained, (iii) the acidifying agent is added to the reaction medium along with, where appropriate, simultaneously, the remaining amount of the silicate, an additional amount of acidifying agent is added to the reaction medium, preferably until a pH of between 4 and 6 is obtained in the reaction medium.
• the separation comprises a filtration and washing operation using a filter equipped with a means of compacting, a suspension having solids content of less than 17% by weight is dried by spraying.
• the drawback of the process is that both the solutions are to be added simultaneously and their rates are to be controlled very critically. Slight variation in rate of addition may affect the pH of the reaction mass and subsequently the quality of silica. Spray drying is required for obtaining silica of desired particle size. This requires high energy input.
• U.S. Pat. No. 6,468,493 (2002) assigned to Chevallier, et al. describes the process for the preparation of precipitated silica, which comprises of the following steps i) an initial stock solution is prepared containing some of the total amount of silicate used in the reaction and at least one electrolyte.
• the silicate concentration in terms of SiO 2 in the said initial stock solution is in the range of 40 to 330 g/l and electrolyte concentration is in the range of 12 to 20 g/l.
• Acidifying agent is added to the stock solution until pH value of between 7 and 8.5 is obtained. To this solution an acidifying agent and remaining silicate solution is simultaneously added in such a way that the pH of the reaction mixture does not change.
• U.S. Pat. No. 6,180,076 (2001) assigned to Uhriahdt et al. describes a method for the preparation of readily dispersible precipitated silica having BET specific surface in the range of 120-200 m 2 /g, DBP index 150-300 and some of the particles are of less than 1 ⁇ m after degradation of the particles by ultra-sonication.
• the particles are readily dispersible having degradability ratio wK ⁇ 3.4 ⁇ m.
• the wK coefficient defined as a ratio of peak height (maximum) of non-degradable particles in the range of 1 to 100 ⁇ m and a maximum of the degraded particles in the range of ⁇ 1 ⁇ m after ultrasonic treatment at 100 W for 4.5 minutes.
• the precipitated silica is obtained by the reaction of alkali metal silicate with mineral acid at a temperature from 65° to 95° C. at pH of 7.0 to 11.0 with continuous stirring. The reaction is continued up to a solids concentration of 40-110 g/l and the final pH is adjusted to a value between 3 and 5. The product is filtered, washed and then dried. Reaction as per this invention is carried out in two steps: 1) Addition of water glass and acid for 15 to 25 minutes followed by interruption of the addition for 30 to 90 minutes. 2) Addition of water glass and acid for 50 to 70 minutes. The total reaction time is 130 to 140 minutes. The particle size obtained after ultra-sonication at 100 W for 4.5 min as per some of the embodiments is ⁇ 5 ⁇ m. The time of reaction is comparatively longer and the particle size without ultra-sonication has not been mentioned.
• the added quantity of acidifying agent is preferably such that 80 to 99% of the added quantity of M 2 O is neutralized.
• the reaction mixture is aged for 1 to 60 minutes.
• an additional quantity of acidifying agent is added to the reaction mixture to reach the pH between 3 and 6.5.
• U.S. Pat. No. 5,851,502 (1998) assigned to Turk et al discloses that precipitated silica can be prepared by introducing water into a precipitation vessel, adding water glass until an alkali value is between 5-15 and simultaneously adding water glass solution and sulfuric acid until the pH of the reaction mass is around 8.5, interrupting precipitation for some time and then further continuing the neutralization with acid until the pH of the slurry is ca. 4.
• the drawback of the process is that both the solutions are to be added simultaneously and their rates are to be controlled very critically. Spray drying is required to obtain desired product, which required high-energy input.
• Indian Patent No. 176707 (1996) assigned to Mody, et al. discloses a process for the preparation of precipitated silica at ambient temperature using hydrochloric acid.
• the process involves preparing aqueous solution of sodium silicate having 1 to 1.3 N Na + ion concentration, adding 14-16% hydrochloric acid to the said solution at constant rate over a period of 10-60 minutes under continuous stirring to bring down the pH of the resultant mixture to around 10.8, continuing addition of the same acid for 3-6 hours to bring down the pH between 3 and 4 to obtain the precipitated silica. Finally, separating, washing, drying and pulverizing the said precipitated silica by known methods.
• the drawback of this process is that complete neutralization of alkali metal silicate solution at ambient temperature takes very long time, which makes process uneconomical. Besides, the process is technically feasible only with hydrochloric acid and not with other acids.
• U.S. Pat. No. 5,342,598 (1994) assigned to Persello describes a process wherein the silica particulates are prepared by simultaneous addition of sodium silicate and a diluted acid into a dispersion of colloidal silica, under continuous agitation. At the completion of the reaction the pH of the suspension is between 3 and 7.
• colloidal silica is used as a nucleating agent and is separately prepared by heating a sodium silicate solution at around 75° C. and adding acid until the final pH is around 9.5.
• the drawback of the process is that both the solutions are to be added simultaneously and their rates are to be controlled very critically.
• dilute colloidal silica is needed for precipitation of silica.
• U.S. Pat. No. 5,094,829 (1992) assigned to Krivak et al. discloses a method for the preparation of reinforced precipitated silica having BET specific surface in the range of 220 to 340 m 2 /g with pore diameter of 9 to 20 nm.
• the process involves the neutralization of alkali metal silicate solution in multiple stages of precipitation and aging. About nine stages of reactions are involved where in certain stages the reaction mixture is brought to pH below 7 and again pH is raised to alkaline condition. Further precipitation by simultaneous addition of acid and silicate solution is commenced and again pH is brought to acidic condition. At least three aging steps are involved in the process.
• the reaction in first step is carried out at temperature below 50° C. and then raised to 80° to 95° C.
• the drawback of the process is that the reaction time is more than 160 minutes, which makes the process uneconomical.
• the particle size of the product is not mentioned.
• U.S. Pat. No. 5,034,207 (1991) and U.S. Pat. No. 5,123,964 (1992) assigned to Kerner et al. describes a process for preparation of silica, having BET specific surface in the range of 150 to 350 m 2 /g, bulk density between 60 and 120 g/liter and at least 70% particles having size of 1 to 6 ⁇ m, can be obtained by heating water glass solution at 700 to 80° C. under agitation, adding concentrated sulfuric acid into water glass solution until half of the alkali present has been neutralized, shearing the reaction mixture and optionally raising the temperature at the same time to 86° C. Addition of acid is interrupted for 30 to 120 minutes and acid addition is continued until the pH of suspension is 3 to 3.5.
• the silica suspension is optionally diluted with water and a centrifugal pump and a hydrocyclone separates the coarse particles.
• the silica is separated by filtration, washed and again suspended in water and spray dried.
• the suspension during the reaction is sheared by passing it through shearing unit (e.g. Dispax reactor) and reintroduced from the top of the reaction vessel.
• shearing unit e.g. Dispax reactor
• U.S. Pat. No. 4,495,167 (1985) assigned to Nauroth et al. describes a process for the preparation of precipitated silica having specific surface area higher than 400 m 2 /g, DBP number higher than 300 and more than 99% particles ⁇ 63 ⁇ m.
• the products are prepared by addition of sodium silicate and sulfuric acid simultaneously in such a way that the water present in the reaction vessel has pH in the range of 6-7 at 40° to 42° C., while stirring.
• the reaction suspension is sheared with a turbine type shearing device during the whole course of reaction i.e. 146 minutes.
• U.S. Pat. No. 4,243,428 (1981) assigned to Donnet et al. describes a process to obtained silica by neutralization of a solution of sodium silicate with an acid under conditions which influence the properties of the silica finally obtained.
• the products obtained having a specific surface area ranging between 100 and 600 m 2 /g, which do not vary upon drying, due to the action of a strong acid on an alkaline silicate, characterized by the fact that the addition of strong acid to the alkaline silicate is interrupted one to three times.
• the temperature of the reacting medium is varied during two phases in which strong acid is successively added to the silicate, separated by one interruption.
• Another embodiment of this prior art is the speed at which the acid is added.
• the acid is added to the silicate at a rate varying according to a law such that the residual alkalinity and the concentration of the total silica decrease linearly as a function of the reaction time, in accordance with the following equation in which:
• Embodiments of the invention can use water-soluble alkali metal silicate for the preparation of precipitated silica having fine particle size.
• the alkali metal silicate can be sodium silicate.
• Embodiments of the present invention can also use sodium silicate of SiO 2 :Na 2 O mole ratio in the range of 3.0 to 3.5.
• the present invention can also provide stepwise neutralization of the sodium silicate solution at elevated temperature in the range of 60 to 90° C. using acidifying agents to obtain precipitated silica having average particle size of 5 to 6 ⁇ m.
• Embodiments of the present invention can use dilute solution of mineral acids such as sulfuric acid and acidic silica sol (silicic acid) for the preparation of precipitated silica having desired properties.
• mineral acids such as sulfuric acid and acidic silica sol (silicic acid)
• the invention also provides a process for preparing precipitated silica that is compatible with use as an antiblocking agent for polypropylene and polyethylene films having high transparency, as a reinforcing agent for elastomers like rubber, as a carrier material for various active materials, as an abrasive in toothpaste formulations, or as a thickening and matting agent in paints, varnishes and ink.
• the present invention provides a process for the preparation of finely divided precipitated silica, the said process comprising the steps of:
• the mole ratio of SiO 2 /Na 2 O in sodium silicate used in step (ii) is in the range of 3.0 to 3.5.
• the mineral acid used is H 2 SO 4 with a concentration in the range of 1 to 10 N.
• reaction temperature used in step (vi) is in the range of 60 to 85 degree Celsius.
• the silica sol is prepared while maintaining the temperature and acid concentration is in the range of 20 to 40 degree Celsius and of 1 to 10N, respectively.
• step (vii) the intermittent interruption of acid addition in step (vii) is maintained for a period in the range of 2 to 15 minutes.
• the time of addition of remaining stoichiometric acid to the reaction mixture in step (ix) to attain the final pH in the range of 3 to 5, is in the range of 5 to 20 minutes.
• the rate of cycles, for circulating the reaction mass through a centrifugal pump in steps (vi) to step (xi) is in the range of 15 to 20 cycles per hour.
• the aim of the present invention is directed to provide a process for preparing finely divided precipitated silica having extremely fine particle size.
• the process involves the controlled neutralization of alkali silicate solution with a combination of neutralizing agents viz. mineral acid and stable silica sol at elevated temperature and under controlled conditions viz. concentration of reactants, temperature, time and mode of neutralization.
• the neutralization process is intermittently interrupted for the growth of the particles.
• the reaction mass is continuously agitated to control the coagulation of the particles.
• the process can be carried out in an open vessel and the product can be easily recovered by conventional filtration techniques, dried and pulverized to obtain fine powder.
• the process involves the controlled neutralization of sodium silicate solution in presence of electrolyte as coagulating agent.
• the neutralization of silicate solution is carried out at elevated temperature in the range of 50° to 90° C. by using diluted mineral acid of concentration in the range of 1 to 10N and stable silica sol.
• neutralization is carried out such that 10 to 30% of the total alkali is neutralized and the reaction mass is aged under stirring for a period in the range of 5 to 20 minutes.
• separately prepared stable silica sol is added while agitating the reaction mass and maintaining the reaction temperature.
• the reaction mass is again aged for at least 3 minutes and the remaining mineral acid is added for a period between 5 to 30 minutes.
• the reaction mixture is continuously circulated through a centrifugal pump at the rate of 15 to 20 cycles per hour during the entire course of reaction.
• the BET surface area is determined by the adsorption of nitrogen at liquid nitrogen temperature as described by Brunauer, Emmet and Teller in Journal of the American Chemical Society , Volume 60, page 309 (1938) using surface area analyzer, ASAP 2010C, Micromeritics USA.
• the oil absorption value is determined by following method:
• the values of bulk density is determined by following method
• the method prescribed by the Indian Standard Specification (IS: 1420-1959) for ‘Light Basic Magnesium Carbonate for Rubber Industry’ was adopted to determine the bulk density.
• the apparatus consists of wooden stand, a measuring cylinder with rubber cork. The distance between zero and 250 ml graduation mark is 200 mm. The height of the free fall of the cylinder is 25 mm.
• a pre-weighed quantity (W) of the sieved and dried sample is gently slipped in to cylinder.
• the cylinder with the rubber cork is then assembled in to the wooden stand.
• the cylinder is gently raised to a height of 25 mm in such a way that it did not dash with the upper stop and then released smoothly.
• the timing of rising and releasing were so adjusted that one tap is given at every two seconds. Total 50 taps are given.
• the particle size distribution analysis was done by laser diffraction technique using Mastersizer 2000, Malvern Instruments using dry powder.
• Novel steps employable in practicing the present invention include (i) the process dispenses the need of simultaneous addition of alkali metal silicate solution and mineral acid under stringent control of pH; (ii) the process does not require the need of ion exchange resin to prepare stable silica sol; (iii) the neutralization can be completed within a period of 80 minutes and it obviates the need of long interruption time during the reaction.
• Precipitated silica is prepared same as described in Example-1. The only change made is in electrolyte concentration, instead of 2.45 kg of sodium sulfate, 2.1 kg of sodium sulfate is dissolved to the initial sodium silicate solution.
• the product after drying is jet milled.
• the product having 360 m 2 /g of BET Surface area, 240 g/100 g Oil absorption, 0.1 g/cc bulk density, 5.0 ⁇ m average particle size with 74% particles ⁇ 10 ⁇ m was obtained.
• the jet milling of the product resulted in further improvement in particle size distribution with 84% particles are less than 10 ⁇ m.
• Addition of acidic silica sol is interrupted for 5 minutes followed addition of 3N sulfuric acid for 10 minutes to bring pH of the reaction mixture to 4.
• Reaction mixture is aged for 5 minutes while agitating at 75° C.
• the reaction mixture is circulated through centrifugal pump from the bottom of reaction vessel and put back to reaction vessel from the top at the rate of 16 cycles per hour through out the course of reaction.
• Precipitated silica is separated by vacuum filtration and wet cake was washed until sulfate free filtrate is obtained.
• the cake is dried at 110° C. in tray drier, pulverized and optionally jet milled.
• the product thus obtained has 373 m 2 /g of surface area, 0.09 g/cc bulk density, 5.0 ⁇ m average particle size and 79% particles are less than 10 ⁇ m before jet milling. After jet milling 93% particles of less than 10 ⁇ m were obtained with average particle size of 4.0 ⁇ m.
• Addition of acidic silica sol is interrupted for 5 minutes followed addition of 3N sulfuric acid for 10 minutes to bring pH of the reaction mixture to 4.
• Reaction mixture is aged for 5 minutes while agitating at 75° C.
• the reaction mixture is circulated through centrifugal pump from the bottom of reaction vessel and put back to reaction vessel from the top at the rate of 16 cycles per hour through out the course of reaction.
• Precipitated silica is separated by vacuum filtration and wet cake was washed until sulfate free filtrate is obtained.
• the cake is dried at 110° C. in tray drier, pulverized and optionally jet milled.
• the product having 0.09 g/cc bulk density, 265 g/100 g of oil absorption capacity, 360 m 2 /g of surface area, 81% particles ⁇ 10 ⁇ m and 5 ⁇ m average particle size was obtained before jet milling.
• the particles of less than 10 ⁇ m were increased to 97% after jet milling of the product with 3.7 ⁇ m average particle size.
• Acidic silica sol is prepared by addition of 650 ml of sodium silicate solution containing 160 g of sodium silicate to 292 ml of 3N sulfuric acid at room temperature while stirring. Addition of acidic silica sol is interrupted for 5 minutes followed by addition of 3N sulfuric acid for 10 minutes to bring the pH of the reaction mixture to 4. Reaction mixture is aged for 5 minutes while agitating at 75° C. Precipitated silica is separated by vacuum filtration and wet cake was washed until sulfate free filtrate is obtained. The cake is dried at 110° C. in tray drier and pulverized and optionally jet milled. The product before jet milling has 73% of particles ⁇ 10 ⁇ m and 5.8 ⁇ m average particle size, those have been change to 87% and 4.6 respectively on jet milling.
• Acidic silica sol is prepared by addition of 650 ml of sodium silicate solution containing 160 g of sodium silicate to 292 ml of 3N sulfuric acid at room temperature while stirring. Addition of acidic silica sol is interrupted for 5 minutes followed by addition of 3N sulfuric acid for 10 minutes to bring the pH of the reaction mixture to 4. Reaction mixture is aged for 5 minutes while agitating at 75° C. Precipitated silica is separated by vacuum filtration and wet cake was washed until sulfate free filtrate is obtained. The cake is dried at 110° C. in tray drier and pulverized. The product obtained has 80% particles ⁇ 10 ⁇ m, 5.5 ⁇ m average particle size, 0.09 g/cc bulk density and 220 g/100 g oil absorption capacity.
• Acidic silica sol is prepared by addition of 1.89 liters of sodium silicate solution containing 470 g of sodium silicate to 856 ml of 3N sulfuric acid at room temperature while stirring. Addition of acidic silica sol is interrupted when? for 5 minutes followed by addition of 3N sulfuric acid for 10 minutes to bring pH of the reaction mixture to 4. Reaction mixture is aged for 5 minutes while agitating at 75° C. The reaction mixture is circulated through centrifugal pump from the bottom of reaction vessel and put back to reaction vessel from the top at the rate of 13 cycles per hour through out the course of reaction. Precipitated silica is separated by vacuum filtration and wet cake was washed until sulfate free filtrate is obtained. The cake is dried at 110° C.
• the product has 0.07 g/cc bulk density, 280 g/100 g oil absorption capacity and 390 m 2 /g surface area.
• the particles of less than 10 ⁇ m has increased to 97% from 83% after jet milling of the product with 3.7 ⁇ m average particle size which is 5.1 ⁇ m before jet milling of the product.

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• 2008
  • 2008-04-23 WO PCT/IN2008/000257 patent/WO2008136019A2/en active Search and Examination
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